1. Field of the Invention
This invention relates to a support structure used for a catalyst for purifying exhaust gases of an internal combustion engine, and in particular to a tandem support structure for an exhaust gas purifying catalyst in which two or more honeycombs are disposed in a sleeve.
2. State of the Art
As a catalyst for purifying exhaust gases of an automobile engine, it is common to use a catalyst in which a catalyst carrier comprising activated alumina or the like is formed on a ceramic or metallic honeycomb, and catalyst metal comprising platinum or rhodium is loaded on the catalyst carrier.
The catalyst metal, however, cannot exhibit catalyst activity at temperatures of less than 300.degree. C. Since the temperature of the honeycomb is low immediately after the engine starts, harmful substances are exhausted with almost no purification until the honeycomb is heated and reaches a temperature of 300.degree. C. or more by the heat of the exhausted gases.
In order to improve this matter, it has become common to employ a honeycomb made of metal which is superior in heat conductivity and temperature rising characteristics to ceramics. Further, Japanese Unexamined Utility Model Publication (KOKAI) No. 141632/1988 and No. 83320/1990 disclose a tandem support structure where two honeycombs are fixed at positions separated from each other in a sleeve, and the metal honeycomb in small volume is disposed on an upstream side of exhaust gases, while the ceramic or metal honeycomb is disposed on an downstream side of the exhaust gases. The reduction of the heat capacity of the honeycomb on the upstream side allows the temperature of the honeycomb on the upstream side to rise rapidly. Besides, catalyst design freedom is increased. For example, it becomes possible to load a larger quantity of catalyst metal which exhibits high activity at low temperatures on the honeycomb on the upstream side.
Further, the above publications and Japanese Utility Model Publication (KOKAI) No. 111124/1989 disclose a support structure in which a metal honeycomb makes only a partial contact with a sleeve, and a support structure in which a metal honeycomb is fixed to a sleeve by way of a heat insulating cushioning material. The employment of these support structures prevents heat radiation caused by the heat transmission between the metal honeycomb and the sleeve. Therefore, the rise of the metal honeycomb temperature is promoted, and the capacity of purifying exhaust gases at the beginning of an engine operation is increased.
Even the tandem support structures described in the above, however, do not have sufficient capacity of purifying exhaust gases at low temperatures immediately after an engine starts, because of the following problems: Even when the metal honeycomb having small heat capacity is disposed on the upstream side of exhaust gases, heat radiates due to the heat transmission at connecting parts of the metal honeycomb and the sleeve. Further, the heat insulating cushioning material has problems in heat resistance and durability in itself, and therefore can be used only for very limited parts. In addition, the employment of the heat insulating cushioning material causes the increase in the whole size of the support structure, production costs, and required space for installing the support structure.